1. Field of the Invention
This invention provides an enhanced rotary face seal for roller cone rock bits. The new seal has pins which positively prevent rotation of the seal ring with respect to the seal ring carrier.
2. Description of the Related Art
Modern, premium roller cone rock bits utilize sealing systems to prevent the loss of lubricant from the roller cones. The seal system also prevents the abrasive laden drilling fluid outside the bit from entering into, and failing the bearing system of the rolling cones.
There are two basic types of sealing systems in common use in rolling cutter drill bits. In most drill bits, an elastomeric packing ring provides the seal between the rolling cone and the bearing system. These bits utilize an elastomeric compression type sealing system, and have adequate performance in most drilling applications. For rock bits used in very severe bit applications, however, rotary mechanical face seals are disposed between the rolling cone and the bearing to provide the seal.
Rotary mechanical face seals are generally made up of two flat sealing faces which are designed to maintain a thin film of lubricant between the sealing faces. As the sealing surfaces rotate relative to each other, they are urged together at a carefully controlled force by one or more energizers as shown for instance in U.S. Pat. Nos. 5,040,624, 4,838,365, and 3,761,145.
Although generally more expensive than elastomer seals, mechanical face seals are able to assure a level of performance in rock drilling bits which easily justifies the higher cost. Most mechanical face seals, also known as rigid face seals used in rotary rock bits are made from stainless steels and have sealing faces which are manufactured to be flat and smooth. These faces mate together to form a planar, annular sealing interface. These seals are usually made with one or two sealing rings with a gradually tapered shape adjacent to the sealing interface at the lubricant side. This creates a diverging geometry which provides preferential access for lubricant to enter into the sealing interface. As abrasives wear the outer periphery of the sealing interface, the diverging geometry also facilitates inward movement of the sealing interface to maintain the contact width.
Mechanical rigid face seals have become the seal of choice for rock bits used in the most severe drilling environments, due to the operating limitations of elastomers as dynamic seals. Rigid face seals are typically manufactured from materials which readily tolerate the thermal, chemical and mechanical attack of severe drilling environments. The seals provide a higher level of reliability than elastomer seals in rock bits and are capable of extremely long runs without significant loss of lubricant.
It is important to maintain a lubricant film between the two sealing faces. Oftentimes in operation, however, the film becomes too thin and frictional contact between the sealing faces will cause high torques on the seal faces. These high torques can cause failure of the systems which hold the seal in place. For instance, if elastomeric energizers are transmitting the torque, they may slip. A small amount of slippage can cause excessive wear on the elastomer energizers, leading to an early failure.
Even when coil spring energizers, such as shown in U.S. Pat. No. 4,838,365, are transmitting the torques, it is possible, under some circumstances for the coil spring energizers to fail. When the operating torques become too high, the shear forces on the coil springs can cause them to yield. Once any one of the springs yield, the seal assembly loses its ability to move in response to volume changes in the lubricant near the seal, leading to rapid seal failure. The key to the proper operation of rigid face seals for rolling cutter drill bits lies in their ability to accommodate the lubricant volume changes near the seal, as described in U.S. Pat. No. 4,516,641. In non-rock bit applications, rigid face seals do not have to deal with this peculiar volume compensation problem. The unique design requirements for rock bit volume compensating rigid face seals are such that they are in a unique class of rigid face seals. There are many superficial similarities between volume compensating rigid face seals for rock bits and non-rock bit face seals. However, the diverging design requirements of the two groups tend to make them non-analogous.
For example, coil springs are often used in rigid face seal applications other than for rolling cutter drill bits. To prevent torque from being applied to the springs, a plurality of pins are often interspersed with the coil springs. These pins allow free axial movement of the seal faces while transmitting all the face torque. It is undesirable in these designs for the springs to carry any part of the face torque because torque loading can profoundly affect the springs' ability to energize the seal faces. The wire coil of the spring may bind or `hang` against the comer of the spring bore, changing the springs' force/deflection characteristics.
A non-rock bit face seal design incorporating coil springs for energizers and pins for torque transmission is disclosed in U.S. Pat. No. 4,261,582, herein incorporated by reference. The pins and their mating bores typically have diameters sized such that all the torque load is transmitted from the seal rings through the pins. No torque is carried by the coil springs. Even in designs where pins and springs are combined, as shown in U.S. Pat. Nos. 4,215,870 and 5,080,378, the components are arranged such that the springs never transmit any of the torque load from the seal ring.
In all the prior non-rock bit mechanical face seal designs known to the inventor of the present invention, great care is taken to assure that no torque is carried by coil spring energizers.
In rock bits, however, coil spring energizers are able to successfully carry torque. The events which normally lead to high face torques in volume compensating face seals in rock bits also tend to relieve the coil springs of their energization duties during these events. This happens because the pressure force on the seal face during an `onward loading` volume compensating event causes the springs to extend and also causes the axial face load to increase. Under this condition, the force contribution from the spring is not necessary for an effective seal of the seal faces. As soon as the event is past, the face torque rapidly decreases as the springs retract. When the pressures are finally balanced, the spring returns to its centered position.
The coil spring is designed such that the thickness of the wire in the coil is greater than the gap between the seal and the cutter bore when the spring is in its centered position. This prevents the wire from `hanging` on the lip of the spring cavity in the cutter from normal operating torques.
However, it has been observed that face torques may sometimes exceed 200 inch-pounds in these bits. At this torque level, the wire in coils can yield, effectively disabling the spring as an energizer. The present invention provides a means to prevent this spring yielding.